The present invention relates to methods for heat treating ferrous parts and more particularly, to improved processes for carburizing and/or bright hardening or annealing such parts in integral quench or vestibule furnaces.
Heat treatment of ferrous parts for the purpose of improving the metallurgical characteristics thereof has been practiced for many years. It is well known to carburize, bright harden and anneal such parts in various types of furnaces to which an appropriate atmosphere is supplied and maintained under particular temperatures. With respect to the carburization of ferrous parts, it has been common practice to supply an atmosphere comprised of "endothermic" gas (40% hydrogen, 40% nitrogen and approximately 20% carbon monoxide with minor or trace amounts of carbon dioxide and water vapor) to the furnace hot zone together with an enriching flow of natural gas or methane. In vestibule type furnaces, the endothermic gas provides an atmosphere which together with the "spike" of enriching natural gas enables carburization of steel parts and by virtue of utilizing relatively high flow rates of endothermic gas, e.g. 300-400 SCFH, a flow of endothermic gas from the hot zone to a vestibule will maintain the vestibule in a condition which permits the safe carburization of ferrous parts.
Endothermic gas is produced by reacting natural gas with air under controlled conditions in a generator. As a consequence of the sharp increase in the cost of natural gas in recent years, the cost of carburizing with atmospheres comprised of endothermic gas has also substantially increased. Consequently, techniques for avoiding the use of carburizing atmospheres based on hydrocarbon sources have been sought. Alternate atmospheres, however, must permit operation of carburizing and other heat treating furnaces in a safe manner and at the same time must be effective in performing the desired heat treatment function, e.g. carburizing, bright hardening, etc.
It has been proposed to substitute an inert gas such as nitrogen for endothermic gas and add methane and an oxidant such as carbon dioxide or air to the nitrogen carrier gas, which mixture is introduced into the hot zone of a carburizing furnace. Such a process is described in British patent specification No. 1,471,880 which indicates that the constituents of the carrier gas mixture are selected so as to establish a carbon monoxide level between 3.9 and 10.7% in the hot zone. This reference states that various substances such as gaseous oxygen, carbon monoxide, carbon dioxide, water vapor, and mixtures thereof may be utilized as an oxidant. Carbon dioxide is clearly preferred as this reference indicates that CO.sub.2 enables appropriate surface carbon concentrations to be obtained with a high nitrogen dilution of the carburizing atmosphere. However, by so utilizing nitrogen-rich carrier gas flows, the reactions occurring in the furnace hot zone become relatively sluggish due to the presence of this large volume of inert gas which absorbs heat and slows reaction kinetics favorable to carburization of ferrous parts. Consequently, a relatively high flow of oxidants (as indicated in Table II of this reference) is required to develop reaction kinetics which will enable carburization at temperatures of approximately 1700.degree. F. Consequently, relatively large flows of hydrocarbon (CH.sub.4) are also required to overcome this oxidant and prevent decarburization. In addition, the use of a nitrogen carrier gas in carburizing processes also tends to reduce carbon monoxide and hydrogen levels of the furnace atmosphere substantially below those levels typically provided by use of endothermic gas, namely 15-20% CO and 40% H.sub.2. With the resulting low concentrations of CO, namely 3.9-10.7% as taught by this British specification, the reactions which permit and best facilitate carburization simply do not occur to the extent desired. Also, low hydrogen levels decrease the capacity of the furnace atmosphere to produce bright surfaces.
In order to avoid the aforementioned disadvantages of utilizing carrier gas based carburizing atmospheres, it has been proposed in U.S. Pat. No. 4,049,473, which is assigned to the assignee of the present invention, to introduce an inert gas into a furnace vestibule and to supply a gaseous carbon source without a carrier gas to the hot zone of the furnace. By so inerting the furnace vestible, carburization may safely proceed in the hot zone. The inevitable leakage of air into the furnace hot zone will generally supply adequate oxidant to enable desired carburizing reactions to proceed. However, it has been found, particularly with extended carburizing cycles, e.g. 8.0 hours or so, that nitrogen introduced into the vestibule will diffuse into the hot zone and will tend to inhibit reaction kinetics therein. Consequently, it has been found that the carburizing atmosphere may exhibit relatively low carbon monoxide levels, e.g. 7-12% and the desired reaction kinetics are not always achieved. It has been proposed to introduce a controlled flow of air into a furnace hot zone to provide predetermined oxidant supply; however, as air is comprised of 79% nitrogen, this approach has not proven particularly successful when natural gas is relied upon as the gaseous carbon source. One consequence of utilizing a "sluggish" atmosphere in the hot zone of a carburizing furnace is that unreacted methane will exist therein and direct reading control equipment will indicate that a relatively high carbon potential exists in the furnace. However, the unreacted methane is not contributing to carbon added to the ferrous parts or workpieces and thus, difficulty in obtaining desired carbon levels in the workpieces is encountered. Although it is known to utilize carbon dioxide as an oxidant, it is believed that CO.sub.2 tends to remove carbon from the parts undergoing treatment, i.e. decarburize such parts, before reacting with free carbon in the atmosphere to generate carbon monoxide. Consequently, additions of carbon dioxide to a carburizing furnace hot zone are not considered to be effective solutions to the problems of safely maintaining desired, reactive carburizing atmospheres.